Infra-red devices and methods



Y. A. ROCARD ET-AL INFRA-RED- DEVICES AND moms April 28, 1959 Filed Nov.'50, 1953 3 Sheets-Sheet 1 INVENTORS 11 65 A Pow/P0 BY Bt'R/Yl/IRD t.Barn s M 7% a? 41-7 INEYS N 6 8 m WATT Q SIGNAL NOISE April 28, 1959 Y.A. ROCARD ETAL INFRA-RED DEVICES AND METHODS Filed Nov. 30, 1953 :sSheets-Sheet 2 INVENTORS has A? P0020 BY BERN/MR0 B4Rras ATTORNEYS wwvwINFRA-RED DEVICES AND METHODS Filed Nov. 30, 1953 3 Sheets-Sheet 3INVENTOR. Yves A Poamm By mo [I 340725 United States Patent INFRA-REDDEVICES AND METHODS Yves A. Rocard, Paris, France, and Bernhard E.Bartels,

Glenwood Landing, N.Y., assignors, by mesne assignments, to HuppCorporation, Cleveland, Ohio, a corporation of Virginia This inventionrelates to improvements in the production of infrared responsive cells,and particularly in such cells of the evaporated layer type.

Infrared responsive cells of the evaporated layer type .have heretoforebeen produced as disclosed, for example,

in US. patent to Cashman No. 2,448,516, and as described in vol. 115 atpage 1 of the January 1952 issue of Science in an article entitledPhotoconductive Cells for Detection of Infrared Radiation, and vol. 159at page 818 of the June 14, 1947, issue of Nature in an article entitledLead Sulfide Photoconductive Cells. Such cells are, however, relativelyunstable, with low sensitivity and low signal-to-noise ratio. The priormethods of producing such cells are not adapted to producing cellshaving predetermined reproducible characteristics and this results inhigh production costs due to the high percentage of unsatisfactorycells.

It is, accordingly, an object of this invention to provide a new andimproved method of producing photocells of improved response to infraredradiations, together with improved stability, high sensitivity, highsignal-tonoise ratio coupled with a low time constant.

Another object of the present invention is the provision of an improvedmethod of producing such cells having predetermined predictablecharacteristics and hence providing for relatively low cost massproduction.

It is also an object of this invention to provide a photocell having anew and improved structure.

A further object of this invention is to provide a photocell of novelcooling means.

Other objects will become apparent in the disclosure herewith inconjunction with the accompanying drawings and the appended claims andin which:

Figure 1 is a sectional view of a complete photocell;

Figures 2 to 9 are views of the components of the cell of Figure 1 toillustrate steps in the assembling of the components;

Figure 10 is a sectional view of the photocell at one step in itsmanufacture;

Figure 11 is showing by way of example, of response curves of a specificlead sulfide photocell under particular temperature conditions;

Figure 12 is a sectional view of an assembly for cooling a photocell;

Figure 13 is a wiring diagram of one form of testing amplifier and ofother test equipment; and

2,884,345 Patented Apr. 28, 1959 Figure 14 is a plan view of an electricoven, and shows heating zones and a photosensitive cell in the heatingzones.

Figure 1 shows a complete infrared responsive cell 10, which comprises abody of a suitable glass which defines an inner thimble 12, and an outerenvelope 14, integrally connected with the thimble to define a chamber16-. A layer 18, of a radiation sensitive lead sulfide sits on top ofthe thimble, and wires 20 and 22, respectively, are connected to thesensitive layer and extend through the envelope at suitable seals 24.The wires 20 and 22 are of tungsten or of other suitable materialscapable of being fused with the glass. The upper end of the body isdepressed, as indicated at 27, to form a relatively thin window portionof the body. The body is formed in steps of taking a glass tube 26(Figure 2) of an external diameter of about 30 m., and of a wallthickness of 1 mm., and a glass plate 28 (Figure 3) about 0.5 mm. thickand of a diameter of about 25 mm. and fusing them together to make theouter protective envelope 14. An indentation is made in the plate 28 tostretch the glass and make the thin window 27.

The inner glass unit or thimble 12, is formed of a glass tube (Figure 5)having an outside diameter of about 18 mm. and 1 mm. thick, and theupper end of the tube is sealed with a flame, and during this operationthe two wires 20 and 22, are embedded in the glass, as indicated at 30(Figures 5 and 7). The glass is next ground to expose the upper ends ofthe wires, as indicated at 32 in Figures 6 and 8. The wires are thenground with a very fine abrasive and polished until the ends areperfectly fiat and free from scratches. This is a very importantoperation because any roughness at these points will cause noise and sodecrease the signal to noise ratio in the cell.

Electrodes are next applied to the exposed tungsten contact points todefine lines 34 and 36, respectively (Figure 9) and it is essential thatthese lines be parallel and of even width. The effective area of thesensitive material is measured between these two lines. The optimum areais 11 x 4 mm. although any other area can be employed, and cells withsensitive areas of 5 x 5, 4 x 4 and 3 x 3 mm. have been produced andhave "operated satisfactorily. Of course, with smaller cell areas theamount of total energy impinging on the sensitive area is less and theoptimum characteristics may decrease.

The tungsten wires are connected to the sensitive layer through theelectrodes 34 and 36. The electrodes iare preferably formed of a depositof Aquadag which is a colloidal graphite-suspension in an aqueous ornonaqueous vehicle.

As shown in Figure 10, the thimble and the outer 'p'rotective envelopeare assembled and sealed t0gether,f and a capillary tube 38 communicateswith the chamber-16: This tube has an external diameter of 6 mm. and aninternal diameter of 1 mm. This tube provides for'iithe introduction ofthe powder used in the production ofijthe sensitive layer 18,, for theintroduction of oxygenigind for the exhausting of gas from the chamber,and is removed, and its entry point sealed ofi when formation of thecell has been completed.

Moisture, even in the smallest amounts, will cause unpredictableoperation of the cell and every precaution must be taken to make certainthat the materials placed in the cell are perfectly dry.

Preferred methods for the preparation of the lead sulfide powder for thecell are now described. For example, in a wet method of lead sulfidepreparation, purified hydrogen sulfide is passed into a 1 molar solutionof lead nitrate or lead acetate or the like. The pH of this solutionwill be approximately 5.55 for the lead acetate and 3.10 for the leadnitrate. Preferably the amount of hydrogen sulfide used is calculated sothat no more than about onethird of the lead salt present in thesolution will combine with the hydrogen sulfide to form a precipitationof lead sulfide. The undesirable impurities are precipitated in thisfirst step. The solution is filtered and this first precipitation isdiscarded. The concentration of lead salts of this solution will then beabout two-thirds molar and will have a pH of about 5.59 for lead acetateand 3.32 for lead nitrate. Hydrogen sulfide is again introduced in themanner and quantity described above and the lead sulfide produced fromthe second precipitation is filtered and dried. The second precipitationcontains the least impurities and is accept-able for use in theproduction of infrared sensitive layers. The remaining solution willhave a lead salt concentration of one-third molar and a pH ofapproximately 5.70 for the lead acetate and 3.70 for the lead nitrate.This remaining solution is discarded because the remainingunprecipitated lead salts contain undesirable impurities which interferewith the photosensitivity of the final product.

A dry method for preparing the lead sulfide has the advantage ofyielding a moisture free product, and in this method purified metalliclead is evaporated into an atmosphere of hydrogen sulfide. A vacuumfurnace or mufile furnace can be used. The lead sulfide powder iscollected and placed in a desiccator until required. Another method ofpreparing dry lead sulfide powder is to react purified lead metal withpurified sulfur in stoichiomet-ric proportions in a vacuum. Theresultant product is collected and stored in a desiccator.

A preferred method of producing the lead sulfide sensitive area withinthe cell is as follows:

About 5-7 milligrams (depending on the cell size) of the lead sulfidepowder, which has been prepared in the manner described above, is passedinto the chamber 16 of the cell body, through the tube 38, and isinitially in zone 1, indicated in Figure 10. The chamber 16 is nowevacuated down to pressure of or 10- mm. of mercury. The lead sulfide isnext transferred from zone 1 to zone 2, indicated on Figure 10 bysublimation. To effect this transfer, the cell is placed in a small ovenas shown in Figure 14. The temperature of the oven in the region Ashould have a minimum value of 580 degrees centigrade, while thetemperature in region B should not exceed 550 degrees centigrade.

The lead sulfide now at zone 2 is oxidized by introducing oxygen whilethe temperatures in region B are still about 580 C. The amount of oxygenused is such that from 1-50% of the lead sulfide deposited on the areabetween the electrodes is oxidized. Approximately 30% oxidation of thelead sulfide layer is preferable. The amount of oxygen used isdetermined by calculating the oxygen required to oxidize approximately30% of the lead sulfide in the cell envelope and proceeding as follows:I

The volume of the vacuum device used in the cell manufacturing procedureis determined and a measured amount of oxygen is introduced into thesystem. The resulting pressure of the oxygen in the vacuum device isrecorded. The reaction between the oxygen and the lead sulfide in thecell will cause a change in the pressure of the oxygen in the vacuumunit. The oxidation procedure is allowed to proceed until the change ofpressure in the system indicates that a sufficient amount of oxygen hasreacted with the lead sulfide, so that approximately 30% of the leadsulfide has been oxidized and optimum sensitivity results.

The oven 40, of Figure 14, may be of the form of a tube 42, upon whichis placed a first heating coil 44 of suitable wire to produce thetemperature desired in region A, and a second heating coil 46, isdisposed about the tube to produce the temperature desired in region B,and suitable independent thermocouple means, such as is indicated at 48,are inserted in the oven from each end so that the operator may observethe temperature in each region. Conventional means may be employed toautomatically control current flow to the heaters to maintain thetemperatures at the desired values.

The oven may be provided with suitable means 50 to position the cellradially within the oven.

When the control tests which will be described in detail hereinafter,indicate that the oxidation procedure should be terminated, the cell isagain evacuated down to 10- or 10- mm. and the cell envelope is heatedso that all of the sublimated lead sulfide remaining around the insideof the envelope is transferred to zone 2. The desired portion of theenvelope may be heated by a handheld gas burner or by the properdisposition of the cell in the oven. The next step is to heat theenvelope so that the lead sulfide is transferred by sublimation fromzone 2 to zone 3, indicated in Figure 10. During this transfer of thematerial, the temperature of zone 3 should be higher than 100 C. toprevent simultaneous condensationof sulfur vapor. A temperature of 150C. has been found satisfactory especially for photocells to be sensitiveat room temperature. At the completion of this transfer, the efiectivelead sulfide will be on the area defined between the electrodes 34 and36 and in conductive connection with those electrodes. Oxygen is againintroduced into the cell at a pressure of 0.2 mm. The cell is againplaced in the oven, and its temperature is raised until the temperaturein zone 2 is 500 degrees centigrade, and the temperature at zone 3,should be no more than 400 degrees Centigrade. The cell is maintained atthese temperatures, and with the difference of temperature given above,for from about one half to five minutes, depending on the size and shapeof the cell. The cell is then cooled rapidly to room temperature byinterrupting the current to the heating coils and by directing a blastof cold air internally of thimble 12, and against the upper end of thethimble to cool the layer of radiation sensitive material now in zone 3.

An important distinguishing feature of the improved method of thepresent invention resides in the separation of the sublimation andoxidation operations into distinct steps permitting much greater controlover activation of the sublimed lead sulfide and resulting in cells ofsubstantially identical characteristics.

After the cell has been cooled, it is then evacuated down to 10* or 10mm. and its resistivity is measured. The

' dark current for one volt applied to the cell may be approximately 10microamperes as a maximum. The photocurrent is then measured when thecell is irradiated for example by a black body radiator operating at atemperature of 300 degrees centigrade, and having an emitting orifice ofone centimeter square, and placed at a distance of approximately 27 mm.from the cell. The photocurrent should then, in this example, be morethan three microamperes.

If the results of this testing procedure indicate that the cell issatisfactory, the cell is completed by flame sealing the capillary tube38, close to the base of the cell. If the tests indicate that the cellhas not met the standards for light and dark currents, the cell isevacuated to l" mm. and heated at 200 C. for l-2 minutes, immediatelycooled and retested. This procedure can be repeated again if the darkcurrents still are not satisfactory. The above procedure increases theresistance of the cell, so that optimum characteristics will result.

Moisture must be excluded at all steps for, while moisture may make thephoto-sensitive material more sensitive under certain conditions, itmakes the operation of the cell unpredictable.

In Figure 11 there is shown the spectral sensitivity curves of the leadsulfide cell prepared in the manner described hereinbefore. The cell wasirradiated by a black body having an emitting orifice of one centimetersquare and placed at a distance of 80 cms. from the cell, and the inputto the cell was modulated at 400 cycles per second so that the output ofthe cell could be measured at the output of an amplifier having a bandpass within the range of 50 to 5000 cycles per second, and so that thenoise range of the cell could be measured.

The solid line I gives the output response of a given lead sulfide cellheld at a temperature of degrees centigrade; the solid line II gives theresponse of the cell when held at a temperature of minus 80 degreescentigrade; and the solid line III gives the response of the cell whenheld at a temperature of minus 180 degrees centigrade. The curves alsoshow how the peak of the response is related to the irradiation in termsof wave length in microns.

The time-constant of the lead sulfide cell made in the mannerhereinbefore described is of the order of 10 seconds when the cell is ata room temperature of 20 degrees centigrade. The signal to noise ratioranges from about 300 to 1 for a grade C cell to as high as 500 to l fora grade A cell.

The working spectral sensitivity of the cell is to wave lengths of about3 microns at room temperature and to about wave lengths of 4.5 micronsat minus 180 degrees centigrade.

In Figure 12 there is shown one means for holding the lead sulfide cellat a low temperature. The lead sulfide cell 10 has a portion of the wallof the thimble 12, treated to present a ground surface, as indicated at62. A body 64, is of a double-wall construction, as in the familiarDewar flask in which there is a vacuum in the space between the twowalls, and a shank portion 66, of the body has a ground glass andportion and is entered in the thimble 12, to make a tight fit with thesurrounding ground glass portion of the thimble. The body 64, also has abowl portion 68, communicating with the shank, and when liquid air ispoured into the bowl it flows into the interior of the thimble to holdthe radiation sensitive material 18 at the desired low temperature.

When the body 64 and the communicating interior of the thimble arefilled with about 200 cubic centimeters of liquid air the cell willoperate at a temperature of about minus 180 degrees centigrade for anhour without recharging of the device with liquid air.

Figure 13 shows diagrammatically an amplifier 72, connected to a cellsuch as has been described hereinbefore to amplify the output of thecell with a gain of the order of one million and a band pass of from 50to 5000 cycles per second. In addition to the amplifier, the diagram ofFigure 13 shows means for indicating the photo-cell current and theoutput current of the amplifier, etc., such as are used in checking thecondition of the cell as it is being made, and mentioned hereinbefore asthe control checking of the cell before it is finally sealed and ashaving met the desired requirements for the cell.

During the sensitizing of the lead sulfide layer of the cell, and whenthe cell has been completed, the factory testing procedure includes theuse of a black body 74, at a temperature of 300 degrees centigrade andwith an emitting orifice of one centimeter square and placed at adistance of cms. from the cell. This irradiates the cell 10, through aradiant energy beam chopping device 76 which passes pulses of energy ata frequency of 400 cycles per second to the cell. The low choppingfrequency is used so that the cell is tested under its poorest serviceoperating condition rather than at its optimum frequency of 800 cyclesper second at which the ratio of signal to noise is markedly higher. Thepolarizing voltage is volts.

Voltage is applied to the cell from a suitable source of DC. voltagethrough voltage regulator means and through a filtering network 78. Theapplied voltage is indicated by a meter 80 and the photocell current, bya meter 82 while the cell is disconnected from the amplifier by a switch84 in the first control tests and the chopper is not being used.

In the subsequent test, the switch 84 is closed and the chopper 76 isoperating. The output of the cell is fed to the control grid of thefirst pentode tube 86; the amplified output from this tube is fed to thecontrol grid of a second pentode tube 88; and its output is fed to thecontrol grid of a third pentode tube 90. The anode circuits of the tubesare fed from a suitable source of DC. voltage through a terminal 96.Bandpass filters are provided in the stages of the amplifier bycapacitanceresistance networks 98 and the stages areresistance-capacitance coupled. A current indicating instrument 100, maybe connected in the output circuit of the amplifier through a bridgenetwork 102 of rectifiers, so that a sensitive D.C. meter can beutilized to indicate the pulsating output.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States LettersPatent is:

1. In the method of producing a photocell in an envelope which containsan interior upstanding thimble therein and with the thimble havingelectrodes thereon, the steps which comprise placing dry lead sulfidepowder in the envelope, then subliming said powder, then condensing thevapor on a surface above the thimble partially oxidizing the condensedvapor, then subliming the lead sulfide again and condensing the vapor todeposit a layer of the sulfide on the electrodes before completing theoxidizing process.

2. In the method of producing a photosensitive lead sulfide cell, themethod which comprises providing a cold envelope in which lead sulfideis collected on a wall, then heating the envelope in the region of thecollected lead sulphide to one temperature while heating the envelope inthe region of a pair of electrodes to a lower temperature so that thelead sulfide is transferred from the wall to the electrodes, introducingoxygen into the envelope after the lead sulfide is transferred to theelectrodes and maintaining the presence of oxygen in the envelope for aperiod of time while the elevated temperature of the regions ismaintained, and finally cooling the envelope rapidly to roomtemperature.

3. In the method defined in claim 2, the additional step of cooling thecell rapidly by directing a blast of cooling medium onto the outersurface of said cell to be effective at the region containing the leadsulfide material.

4. In the method of producing a photosensitive cell, the method whichcomprises the steps of entering a quantity of lead sulfide into a firstzone in an envelope, then evacuating gas from said envelope, thentransferring the second position in said envelope by sublimation bymaintaining the temperature of the envelope in the region of the firstzone at a higher temperature than the temperature of the envelope in theregion of the second zone, introducing oxygen into the envelope whilethe sublimed material is in the second zone while maintaining anelevated temperature in the region of the envelope about the second zonesubstantially constant for a period of time necessary to cause partialoxidation of the lead sulfide in said second zone, then evacuating saidenvelope, heating the envelope in the region of said second zone tocause evaporation of the lead sulfide onto a third zone including spacedelectrodes, then introducing oxygen into the envelope while maintainingthe third zone at an elevated temperature to complete the activation ofthe lead sulfide, and then rapidly cooling the envelope to roomtemperature.

References-Cited in the file of this patent UNITED STATES PATENTS ReifelSept. 27, 1932 Bennett Aug. 8, 1939 Schlesinger Dec. 31, 1940 Eitel Feb.1, 1944 Mayle Nov. 27, 1945 Small Dec. 31, 1946 Snkumlyn Dec. 30, 1947Cashman Sept. 7, 1948 Cashman Sept. 7, 1948 Levinson et al July 12, 1949Ellwood May 16, 1950 Law Feb. 6, 1951 Gibson Mar. 6, 1951 Anderson Apr.21, 1953 Cashman Jan. 10, 1956

1. IN THE METHOD OF PRODUCING A PHOTOCELL IN AN ENVELOPE WHICH CONTAINSAN INTERIOR UPSTANDING THIMBLE THEREIN AND WITH THE THIMBLE HAVINGELECTRODES THEREON, THE STEPS WHICH COMPRISES PLACING DRY LEAD SULFIDEPOWDER IN THE ENVELOPE, THEN SUBLIMING SAID POWDER, THEN CONDENSING THEVAPOR ON A SURFACE ABOVE THE THIMBLE PARTIALLY OXIDIZING THE CONDENSEDVAPOR, THEN SUBLIMING THE LEAD SULFIDE AGAIN AND CONDENSING THE VAPOR TODEPOSIT A LAYER OF THE SUFLIDE ON THE ELECTODES BEFORE COMPLETING THEOXIDIZING PROCESS.